Catalyst components of the Ziegler-Natta type for the stereospecific polymerization of olefins are widely known in the art. The latest developed catalysts for propylene polymerization comprise a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound and often with an external donor which is needed in order to obtain higher isotacticity. One of the preferred classes of internal donors is constituted by the esters of phthalic acid, diisobutylphthalate being the most used. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system is capable of giving good performances in terms of activity, and propylene polymers with high isotacticity and xylene insolubility endowed with an intermediate molecular weight distribution.
Use of some phthalates however has been recently addressed as involving potential toxicity problems and therefore research activity have been devoted to find alternative classes of donor capable of replacing phthalates in terms of both performances and quality of the product.
One of the most interesting classes is that described in U.S. Pat. No. 7,388,061 disclosing esters belonging to the formula R1—CO—O—CR3R4-A-CR5R6—O—CO—R2 in which R1 and R2 groups, which may be identical or different, can be substituted or unsubstituted hydrocarbyl having 1 to 20 carbon atoms, R3-R6 groups, which may be identical or different, can be selected from the group consisting of hydrogen, halogen or substituted or unsubstituted hydrocarbyl having 1 to 20 carbon atoms, R1-R6 groups optionally contain one or more hetero-atoms replacing carbon, hydrogen atom or the both, said hetero-atom is selected from the group consisting of nitrogen, oxygen, sulfur, silicon, phosphorus and halogen atom, two or more of R3-R6 groups can be linked to form saturated or unsaturated monocyclic or polycyclic ring; A is a single bond or bivalent linking group with chain length between two free radicals being 1-10 atoms, wherein said bivalent linking group is selected from the group consisting of aliphatic, alicyclic and aromatic bivalent radicals, and can carry C1-C20 linear or branched substituents; one or more of carbon atoms and/or hydrogen atoms on above-mentioned bivalent linking group and substituents can be replaced by a hetero-atom selected from the group consisting of nitrogen, oxygen, sulfur, silicon, phosphorus, and halogen atom, and two or more said substituents on the linking group as well as above-mentioned R3-R6 groups can be linked to form saturated or unsaturated monocyclic or polycyclic ring.
The examples reported in the document seem to show in general, capability to produce polymers with a broad molecular weight distribution, with a polymerization activity and stereospecificity ranging from very poor (see example 68 and 86) to good performances. Looking at the result of table 1 the catalysts based on diesters of 1,2 diols (examples 1-4) seem in general endowed with an inferior activity/stereospecificity balance with respect to the catalysts based on diester of 1,3 diols. Moreover, by evaluating the series of examples 5, 9-12 and the series of examples 24-26 and 28 it clearly results that the presence of a substituent on the phenyl ring of the benzoic moiety of the donor worsen the catalyst performances particularly in terms of stereospecificity.
Derivatives of 1,2 diols in which one hydroxy group is esterified with a benzoic group and the other is etherified also do not seem very promising. WO2011/106500 describes the use of 1-methoxypropan-2-yl benzoate always in combination with amide esters acting as primary internal donors. The testing of a catalyst based on 1-methoxypropan-2-yl benzoate alone carried out by the applicant reported not completely satisfactory results.
Also not very satisfying results have been obtained by testing another monoether/monoester of 1,2 diol, namely 2-methoxyethyl benzoate. The use of this donor was described in CN101914172A for the preparation of a catalyst used for butene-1 polymerization. The poor results showed for butene-1 polymerization has been confirmed when tested for propylene polymerization. Based on the above it has been very surprising to see that it was possible to improve the class of monoethers/monoesters of 1,2 diols by a specific substitution of the phenyl ring of the benzoic moiety. Such modified structures in fact, can generate catalysts showing a so excellent balance of high stereospecificity and high activity to make them eligible as replacement of phthalate donors.